Phosphate uptake kinetics for four species of submerged freshwater macrophytes measured by a 33P phosphate radioisotope technique

Phosphate (Pi) uptake kinetics were determined in shoot and root tissues for four freshwater macrophyte species, Littorella uniflora, Potamogeton perfoliatus, Myriophyllum alterniflorum and Elodea canadensis, using a radioactive 33P phosphate technique. Collection of plant material in the oligotrophic softwater lake, Lake Hampen, Denmark, where Pi limits macrophyte growth, enabled us to characterize and compare the Pi uptake kinetics and competitive characteristics of the four species in a low level Pi environment. The maximum Pi uptake rates (Vmax), the half saturation constants (Km) together with the affinity at low Pi concentrations (Vmax/Km) were determined by fitting data to the Michaelis-Menten kinetics. L. uniflora showed the highest Vmax/Km in the root tissue and the lowest Km. M. alterniflorum showed the highest and E. canadensis and P. perfoliatus the lowest Vmax/Km in leaf tissue. M. alterniflorum had the highest Vmax and, as the only species, a higher Vmax in leaves than in roots. Surface area explained about half of Vmax in M. alterniflorum leaves. Roots were the dominant organ for Pi assimilation for all species at the Lake Hampen Pi concentrations. Km showed positive correlation to%P content in root tissue. The results indicate that at low lake water Pi concentrations L. uniflora is able to survive on the Pi pools in the sediment porewater. M. alterniflorum showed high affinity for Pi at both low and high Pi concentrations by both roots and shoots, and suggests that M. alterniflorum is a strong competitor at both low and high Pi concentrations.